The χ-Blade

The χ-blade is an ancient weapon of unknown origin, introduced in Kingdom Hearts Birth by Sleep, that can directly unlock Kingdom Hearts. Its light drove many to fight over it in what became the Keyblade War; the war ended in the Keyblade Graveyard and left the χ-blade shattered into seven pieces of light and thirteen pieces of darkness, and the first Keyblades were crafted in its image. Master Xehanort's plan to obtain it has Vanitas pit the Unversed against Ventus to strengthen him. When Ventus and Vanitas fuse back into one being with the χ-blade in hand, the unstable weapon explodes because Vanitas is destroyed within Ventus.